Phase transformations during the reaction heat treatment of Powder-In-Tube Nb$_{3}$Sn superconductors

The phase transformations prior to the superconducting A15 phase nucleation can influence the critical properties of fully reacted Nb$_{3}$Sn superconductors. We describe for the first time the phase transformations occurring during the heat treatment of state-of-the-art Nb$_{3}$Sn Powder-in-Tube strands, based on in-situ synchrotron diffraction measurements. All pure Sn present in the non-heat treated PIT strand is transformed into Cu6Sn5 by solid state diffusion. Subsequently NbSn2 and Cu6Sn5 are partly transformed into a Sn-rich ternary Cu-Nb-Sn phase. When Sn starts to diffuse into the Nb tubes at about 520°C all Sn-containing phases are transformed into NbSn2 and subsequently into Nb6Sn5.